The invention relates to wireless communication systems and, more particularly, to using channel loading statistics to determine whether to search for a new channel in a wireless communication system.
Wireless communication systems allow remote and often portable transceiving devices, e.g., radio telephones (xe2x80x9cmobile stationsxe2x80x9d), to communicate with each other and with stationary transceiving devices, e.g., cellular stations (xe2x80x9cfixed stationsxe2x80x9d) over great distances. FIG. 1 shows a typical wireless network 10, in which several mobile units 12, 14, 16 communicate with two fixed stations 18, 20. The fixed stations 18, 20 communicate with each other, e.g., via radio frequency (RF) signals 22 or via a public switching telephone network 24 (PSTN).
Many wireless networks, including cellular networks such as the Personal Handyphone System (PHS) networks in Japan and the Digital European Cordless Telephone (DECT) networks in Europe, utilize dynamic channel assignment, through which fixed stations with overlapping coverage areas use and reuse preassigned channels. In some systems, such as PHS, the mobile and fixed units employ time division multiple access (TDMA) and/or time division duplex (TDD) communication, which allows each fixed station to communicate with multiple mobile units during a given transmit/receive frame. Each transmit/receive frame may include several discrete time slots, each dedicated to transmitting information between a fixed station and a particular mobile unit.
The fixed stations in many TDMA/TDD-based networks, including PHS networks, may be either public or private. In general, a public fixed station may be accessed by any mobile unit within its range, and a private fixed station may be accessed only by mobile units assigned to it. While public fixed stations typically operate under the control of synchronized internal clocks, the clocks in private fixed stations typically are not synchronized. Moreover, the clocks in private PHS fixed stations are required to be accurate only to 5 ppm; over time the clocks in private fixed stations tend to drift with respect to one another. Because a virtually limitless number of PHS mobile units and fixed stations may exist within a given geographic area, and because PHS networks use Dynamic Channel Assignment, private fixed stations are subject to great variations in the interference they experience from other fixed stations. For example, a carrier that presents relatively little interference when first selected by a fixed station may become too noisy for adequate communication when another fixed station suddenly switches to the carrier or as the transmit/receive frames of other transceivers gradually drift onto each other.
The invention allows the accumulation of channel loading statistics from which a transceiver communicating on one channel can determine whether it should seek another channel, such as when interference arises on the first channel. The transceiver monitors all of the available channels to determine whether any of the channels are idle and accumulates data about the idle channels. The transceiver can use the accumulated data to determine whether any of the channels are idle at any given time. The transceiver is allowed to seek another channel if the data indicates that the transceiver is likely to find an idle channel.
In some embodiments, the transceiver seeks another channel if it determines with a given level of confidence that more than a given number of channels are likely to be idle. The transceiver can determine whether any of the available channels are idle by determining which of the channels has a noise floor level below a given threshold level. In many embodiments, the transceiver compares the noise floor level of each channel to multiple threshold levels to provide several noise levels at which the channel can be considered xe2x80x9cidle.xe2x80x9d The data accumulated by the transceiver may indicate, for each threshold level, how many of the channels have noise and interference floor levels (hereinafter xe2x80x9cnoise floorsxe2x80x9d) below the threshold level. The transceiver also may compile information indicating an average number of channels over a given time period (e.g., 15 minutes) that have a noise floor level below the given threshold level. The transceiver then can apply Gaussian statistical analysis to calculate a minimum value for the average number that ensures with a given level of confidence that more than a given number of channels are idle at any given time. For example, the transceiver can use Gaussian statistical analysis to determine that in a network having 100 channels, at least ten channels on average must be idle to ensure with 99.9% confidence that more than one channel will be idle at any given time. In many embodiments, the transceiver begins searching for another channel only if the data indicates that the average number of idle channels is at least as great as the minimum value.
Many advantages result from the invention. For example, a wireless transceiver may, upon experiencing interference from another transceiver, make an educated decision to seek another channel based on use patterns of the available channels. Since seeking a new channel takes time and can degrade call quality, the transceiver can avoid a futile channel search if it is likely not to find an idle channel. As a result, the invention can lead to increased system efficiency and improved call quality.